Because fo the better coefficient of performance of heat pumps as compared to electrical resistive heating elements, there has been a movement in recent years to purchase electric resistance hot water heaters and then connect the water heater tank to a heat pump so that the heating was actually performed by the heat pump rather than the electric resistance elements in the water heater itself. This produced a good coefficient of performance while at the same time retained the electric resistance heating elements as a backup so that the hot water may still be obtained in event of failure of the heat pump itself. Such an arrangement is illustrated in my earlier U.S. Pat. No. 4,330,309, granted May 18, 1982.
In order to operate these heat pumps, however, it was necessary that a separate thermostat be placed in a heat transfer relationship with the water in the hot water tank to control the heating cycle of the water heating heat pump. Several proposals to place a thermostat in addition to the thermostat in the existing circuitry for the electric resistive heating elements have been proposed. One such proposal is set forth in my earlier U.S. Pat. No. 4,330,309. Another proposal is set forth in U.S. Pat. No. 4,385,723, issued May 31, 1983, and owned by the same assignee as this application. One of the problems with these types of connections is that many governmental codes preclude the use of additional electrical circuitry within the confines of the hot water heater other than that already approved by Underwriters Laboratory for the existing tank circuitry.